Cho ∫124fx−2xdx=1. Khi đó ∫12fxdx bằng :

Chọn A

Ta có $\underset{1}{\overset{2}{\int }}\left[4f\left(x\right)-2x\right]dx=1\iff 4\underset{1}{\overset{2}{\int }}f\left(x\right)dx-2\underset{1}{\overset{2}{\int }}xdx=1\iff 4\underset{1}{\overset{2}{\int }}f\left(x\right)dx-{\left.x^2\right|}_1^2=1$

$\iff 4\underset{1}{\overset{2}{\int }}f\left(x\right)dx=4\iff \underset{1}{\overset{2}{\int }}f\left(x\right)dx=1.$